In general, ink jet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming media. A phase change ink jet printer employs phase change inks that are in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The molten ink can then be ejected onto a printing media by a printhead directly onto an image receiving substrate, or indirectly onto an intermediate imaging member before the image is transferred to an image receiving substrate. Once the ejected ink is on the image receiving substrate, the ink droplets quickly solidify to form an image.
In both the direct and offset printing architecture, images may be formed on a media sheet or a media web. In a web printer, a continuous supply of media, typically provided in a media roll, is mounted onto rollers that are driven by motors. A loose end of the media web is passed through a print zone opposite the print head or heads of the printer. Beyond the print zone, the media web is gripped and pulled by mechanical structures so a portion of the media web continuously moves through the print zone. Tension bars or rollers may be placed in the feed path of the moving web to remove slack from the web so it remains taut without breaking.
In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving web. In continuous-web direct to paper printing, a high pressure roller nip, also referred to as a spreader, is used after the ink is jetted onto the web to spread the ink on the web to achieve the desired print quality. The function of the spreader is to take what are essentially isolated droplets of ink on web and smear them out to make a continuous layer by pressure and/or heat so that spaces between adjacent drops are filled and image solids become uniform.
One difficulty faced in spreader performance is variation in the temperature of the ink on the web as the web enters the spreader. For optimum spreader performance, ink and web temperatures should be substantially uniform prior to entering the spreader and be at a temperature that promotes adherence of the melted ink to the web, minimizes “show through” of the ink through the web, and maximizes ink dot spread. Without additional heating, the ink temperature of a given area on the web as it enters the spreader is dependent upon the mass of the ink that is placed on the area during printing and cooling due to the different positions of the print heads from the spreader. Imaged areas on the web having more ink mass, e.g., secondary color solid fill areas having two or more ink layers, retain more energy, or heat, than imaged areas having less ink mass, e.g., primary color solid fill areas having one ink layer and halftone areas. Ink may also lose heat due to pattern dependent lateral cooling. For example, imaged areas of the web at which, for instance, isolated dots, or lines have been placed may lose energy laterally through the web. The rate of speed of the web is also a factor in ink temperature. For example, the faster that the web W travels between the printing zone and the spreader, the less time the ink has to cool due to convective heat loss, and vice versa. Therefore, without additional heat prior to spreading, lines, halftones, primary color solid fill areas, and secondary color solid fill areas, may all be at different ink temperatures upon entering the spreader.